Hydrogen is used as a feedstock for many chemical processes and has been proposed as an alternative fuel especially for use in fuel cells in stationary and mobile facilities. Steam reforming of hydrocarbon-containing feedstock is a conventional source of hydrogen. Steam reforming of hydrocarbons is practiced in large-scale processes, often at a facility having refinery or chemical operations. Thus, for instance, the large-scale hydrogen plant will likely be able to draw upon the skills within the entire facility to operate sophisticated unit operations to enhance hydrogen production efficiency. An additional benefit of having a large scale hydrogen plant within a facility having refinery or chemical operations is that the steam generated in the hydrogen plant from cooling the steam reforming effluent and by heat exchange with the combustion of waste gases has value to such other refinery or chemical operations. The benefits of practicing steam reforming in large-scale plants are also apparent from the nature of the equipment and process. For instance, steam reforming generally uses very high temperatures, often in excess of 800° C., which in turn requires expensive materials of construction. Furthermore, large-scale hydrogen plants typically provide hydrogen product purity in excess of 99 volume percent with less than 10 parts per million by volume (ppmv) of carbon monoxide.
While the economics of large-scale steam reforming make attractive the shipping of hydrogen from such a large-scale reformer to the point of use, hydrogen, nevertheless, is difficult to store and distribute and has a low volumetric energy density compared to fuels such as gasoline. Thus an interest exists in developing economically and practically viable smaller-scale hydrogen generators to provide hydrogen from a hydrocarbon-containing feedstock for use or distribution at a point proximate to the consumer.
There are a number of practical hurdles for such a smaller-scale hydrogen generator to overcome before it is commercially viable beyond overcoming the loss of economy of scale. For instance, the smaller scale may not support sophisticated operating and technical staff and thus the hydrogen generator must be able to operate reliably with minimal operator support while still providing an economically acceptable hydrogen product meeting purity specifications. Often smaller-scale hydrogen generators face problems that do not occur with large-scale hydrogen plants. An example is that the hydrocarbon-containing feedstocks most often available to smaller-scale hydrogen generators are natural gas and LPG, both of which contain odorants (sulfur compounds) for safety reasons. As sulfur compounds can poison catalysts and may be unacceptable in the product hydrogen, smaller-scale hydrogen generators must incur the expense to remove them. Additionally, smaller-scale hydrogen generators may be stand alone units with no chemical or refinery operation to which steam can be exported.
Consideration has been given to the use of less efficient, but less capital intensive, alternative reforming technology such as partial oxidation/steam reforming, including autothermal reforming. But as a portion of the feed is oxidized in the reformer, efficiency penalties are taken that are not incurred by steam reforming. Accordingly, for partial oxidation/steam reforming to be competitive capital costs for the hydrogen generator must be low, the hydrogen product must meet purity requirements, and the amount of hydrogen produced per unit of hydrocarbon-containing feed must be adequately high.
Partial oxidation/steam reforming, including autothermal reforming, has been extensively studied. In general, studies have shown that the reforming reaction is an equilibrium reaction influenced by temperature and pressure. All other things being equal, lower pressures and higher temperatures favor the production of hydrogen, but higher temperatures necessitate more consumption of fuel, thus are disadvantageous. Similarly, higher ratios of steam to hydrocarbon-containing feedstock favor the production of hydrogen, but the vaporization of water requires heat. Hence, most often partial oxidation reformers use no more than about 3 moles of steam per carbon in the hydrocarbon-containing feedstock.
The reformate from partial oxidation/steam reforming will contain carbon monoxide, carbon dioxide, hydrogen, unreacted hydrocarbon-containing compounds and nitrogen and argon (with air being used as the source of the oxygen-containing gas for the partial oxidation) as well as water. To enhance the efficiency of partial oxidation/steam reforming, the use of water gas shift to convert carbon monoxide and water to carbon dioxide and hydrogen is often used. Processes that have been proposed to remove the remaining carbon monoxide include selective oxidation and methanation.
Membrane and pressure swing adsorption separations can be effective for purifying the hydrogen product since they can remove nitrogen, argon, carbon dioxide, carbon monoxide and unreacted hydrocarbon-containing compounds. However, membrane and pressure swing adsorption systems typically require the gases fed to them to be at elevated pressure. Large-scale steam reformers can tolerate the use of reforming temperatures that are suitable to provide a reformate at pressures suitable for such separations. However such is not the case with smaller-scale partial oxidation/steam reforming units where it is desirable to operate at lower temperatures in order to avoid expensive metallurgy and reduce capital costs. And it is not the case for stand alone hydrogen generators where opportunities to export steam do not exist. Because of the adverse effect of pressure on the efficiency of hydrogen production in these partial oxidation/steam reforming processes, reforming would typically occur at lower pressures, and then the reformate would be compressed to the required pressures. However, additional operating and capital costs are entailed in employing such a compressor. Moreover, membrane and pressure swing adsorption systems can be particularly disadvantages for a smaller-scale hydrogen generator due to loss of hydrogen. The retentate, in the case of membranes, and the purge gas, in the case of pressure swing adsorption, contain unrecovered hydrogen and thus reduce the net hydrogen efficiency (NHE) (heating value of purified hydrogen recovered per unit heating value of hydrocarbon-containing feedstock to the generator). This reduction in net hydrogen efficiency can be deleterious to achieving an economically-competitive smaller-scale hydrogen generator.
Accordingly, processes are sought that yield a hydrogen product of suitable quality, including a very low carbon monoxide concentration; provide favorable economics as compared to shipping and storage of hydrogen produced by a large-scale hydrogen plant; are easily operated with minimal needs for technical sophistication and maintenance.